Support positional variadic task arguments.

[samuel-williams-shopify]

Allow task authors to define a Vec<T> as an unbounded positional argument list. This lets project commands accept natural argument lists such as cargo bake markdown:normalize **/*.md.

Previously, Vec<T> parameters were only available as repeatable named options. The new #[bake(positional)] annotation makes bare values fill a Vec<T> through the end of that task invocation. Since the parser cannot infer whether a later bare value is another task or another list item, callers use :: to start the next task. Named options remain usable alongside the positional list.

The macro validates the annotation and requires the variadic parameter to be the final positional parameter. Task help and author documentation describe the new syntax and chaining rule.